Interpretive Summary: The influence of the continuing rise in atmospheric carbon dioxide level on crops and forests is important since most plants tested have been found to respond to elevated carbon dioxide. One measure proposed to slow the increase in carbon dioxide in the atmosphere is to store it in the soils of crop fields and forest. One little explored aspect of plant response is that of fine root dynamics and how they contribute to soil carbon storage. Fine root growth was followed in a loblolly pine forest for six years to assess their contribution to carbon flow into the soil. Although carbon dioxide enrichment contributed somewhat to carbon movement into soil in this experiment, the magnitude of the effect was small indicating only a modest potential for fine roots to contribute to the storage of carbon in southeastern pine forests.

Technical Abstract:
Efforts to characterize carbon cycling among atmosphere, forest canopy, and soil carbon pools are hindered by poorly quantified fine root dynamics. We characterized the influence of free air CO2-enrichment (ambient + 200 ppm) on fine roots for a period of 6 years (Autumn 1998 through Autumn 2004) in an 18 year old loblolly pine (Pinus taeda) plantation near Durham, NC (USA) using minirhizotrons. Root production and mortality were synchronous processes that peaked most years during spring and early summer. Seasonality of fine root production and mortality was not influenced by atmospheric CO2 availability. Averaged over all six years of the study, CO2 enrichment increased average fine root standing crop (+23%), annual root length production (+25%), and annual root length mortality (+36%). Larger increase in mortality compared to production with CO2 enrichment is explained by shorter average fine root lifespans in elevated plots (500 days) compared to controls (574 days). The effects of CO2-enrichment on fine root proliferation tended to shift from shallow (0-15 cm) to deeper soil depths (15-30) with increasing duration of the study. Diameters of fine roots were initially stimulated by CO2-enrichment but this effect diminished over time. Averaged over six years, annual fine root NPP was estimated to be 163 g dw m-2 yr-1 in CO2 enriched plots and 130 g dw m-2 yr-1 in control plots (P = 0.13) corresponding to an average annual additional input of fine root biomass to soil of 33 g m-2 yr-1 in CO2-enriched plots. A lack of consistent CO2 x year effects suggest that the positive effects of CO2 enrichment on fine root growth persisted six years following minirhizotron tube installation (8 years following initiation of the CO2 fumigation). Although CO2-enrichment contributed to extra flow of C into soil in this experiment, the magnitude of the effect was small suggesting only modest potential for fine root processes to directly contribute to soil C storage in Southeastern pine forests.